This invention pertains to the field of convective heat transfer.
In the field of convective heat transfer, there is in general a tradeoff between heat transfer and pumping power. Power to run a pump or fan to move the fluid involved in heat transfer is usually an expense associated with achieving heat transfer. This is especially of concern in heat exchangers in which the fluid on at least one side is gas such as atmospheric air. Gas side heat exchange is characterized by a relatively small heat transfer coefficient and a relatively small volumetric heat capacity of the gas. Gas side heat exchange designs make up for these drawbacks with large heat transfer surface area and large volumetric flowrate of gas, which together can require a significant amount of power to move the gas. Furthermore, simple fans are frequently inefficient at converting electrical power to gas motion. All of this is especially true when, as is usually the case, there are limitations on the overall space occupied by the heat exchanger. Applications include liquid-to-gas heat exchangers, gas-to-gas heat exchangers, evaporators, condensers, air conditioning and heating equipment, vehicular radiators, heat sinks for electronics, process equipment, electrical generating plants in which the circulating fluid is gas, electrical generating plants which reject heat to the atmosphere, etc. It is also applicable to non-gas heat exchange.
This tradeoff has led to many investigations, both theoretical and empirical, of designs of fins and related geometries. A discussion of this tradeoff is given in xe2x80x9cCompact Heat Exchangersxe2x80x9d by Kays and London. U.S. Pat. No. 5,738,168 also discusses this tradeoff. This patent uses louvers to locally break up the fluid boundary layer and cause mixing of fluid near heat transfer surfaces without causing a large effect on overall pressure drop. Such an approach is typical of the field of enhanced heat transfer. Approaches such as these have resulted in designs of reasonably satisfactory heat exchangers, radiators, etc. However, there is always room for improvement in regard to the tradeoff between heat transfer and pumping power. Such improvement would increase the efficiency, however it might be defined, of any of the various devices employing forced convection heat transfer or even natural convection heat transfer. So far no designs have considered nonuniform distribution of fins as a way of obtaining a more advantageous situation than is obtained with uniform distribution of fins.
Accordingly, it is an object of the invention to achieve, within a constrained geometric envelope of space available for heat transfer surface, increased heat transfer for a given fluid pumping power, or, conversely, reduced fluid pumping power for a given amount of heat transfer.
It is further an object of the invention to achieve similar benefits in natural convection heat transfer, such as a smaller temperature difference between the source and the fluid, for a given amount of heat transfer, using only minor changes in the design of fins, compared to conventional uniformly-spaced fins.
The present invention is a design of fins which, for fixed overall geometrical envelope, produces an improved amount of heat transfer per unit of pressure drop. In contrast to conventional technology, the present invention does not have a uniformly-spaced pattern of fins. Instead, the heat transfer region has at least two flowpaths in parallel. Each flowpath is a series combination of a lower-velocity region and a higher-velocity region, with the lower-velocity region serving especially to accomplish heat transfer by having a significant concentration of heat transfer surface area, and the higher velocity region serving to transport the fluid the rest of the way with relatively little pressure drop by having a relatively small concentration of heat transfer surface area. Thus, most of the heat transfer is accomplished to lower-velocity flow because heat transfer to lower-velocity flow yields a better ratio of heat transfer to pressure drop than does heat transfer to higher-velocity flow. Compared to conventional design, when the geometry is planar the present invention is a replumbing, in at least one place, which changes the series or parallel relationship among various passageways, combined with a shifting of positions of fins in the sideways direction (perpendicular to the fin surface).